Sub-unit vaccines often require the aid of an adjuvant to help boost immune activity. Chemical adjuvants such as aluminium salts and MF59™ have been approved for human use. However, aluminium salts are subject to safety concerns and are incompatible with some antigens. Furthermore, it produces a Th2 type of helper T cell response, which is often inappropriate or insufficient for protective immunity.
The best known adjuvant in laboratory use is Complete Freund's Adjuvant, which consists of killed Mycobacterium tuberculosis suspended in oil. Although this adjuvant is not suitable for human use due to its toxicity, safer adjuvants have been derived from other pathogenic organisms.
The immunostimulatory activity of materials derived from pathogens is believed to reflect the natural host-pathogen interaction. When the antigen-specific immune response evolved, it would have done so in an environment containing adjuvant-active bacterial components. The response to a pure bacterial antigen, injected without adjuvant-active bacterial components, is therefore an artificial situation to which the host would not be adapted to respond.
Components of pathogens are therefore believed to act as “danger signals”, which put the immune system on alert. Examples of adjuvants in this category are components of bacterial capsules, LPS (lipopolysaccharides) from Gram negative bacteria, the glycolipids and arabinogalactans in mycobacteria and the peptidoglycans of spirochaetes. Other known adjuvants include DNA comprising unmethylated CpG dinucleotide motifs, which are relatively rare in vertebrate DNA compared to bacterial DNA, and double-stranded RNA, which mimics the presence of an invading virus.
Polypeptides from pathogens have not received much attention as potential adjuvants. One means to identify adjuvant-active polypeptide sequences from pathogens would be by high-throughput screening, but this approach is essentially random and undirected, such that effort will be wasted on screening polypeptides which are unlikely to function as adjuvants.
Many of the most widely used vaccines consist of whole organisms. These include live organisms that have been rendered safe by attenuating mutations (e.g. tuberculosis and rubella) and organisms killed or inactivated by chemical treatment (e.g. influenza and hepatitis A virus). That these types of vaccines are based on whole organisms presents both advantages and disadvantages. While having all of the components of the pathogen contained within the vaccine is useful for eliciting immune responses against multiple antigens that are structurally similar to those found on the infecting pathogen, some components of whole organism vaccines can cause undesirable side effects. Furthermore, live organism vaccines, although attenuated, can sometimes cause problems in immunosuppressed individuals and have the potential to revert to a virulent state. These disadvantages spurred a movement towards potentially safer, more defined vaccines consisting of partially purified subunits known to be targets for protective immune responses (e.g. tetanus toxoid and influenza haemagglutinin). With the advent of recombinant DNA technology came the ability to produce protein antigens in heterologous expression systems (e.g. hepatitis B surface antigen). In this way, high levels of protein can be manufactured, while eliminating contamination by toxic components of the pathogen. The progression from whole organisms to subunit vaccines has highlighted a need to augment these more purified vaccine components with adjuvants, as vaccines based on live attenuated organisms contain built-in adjuvants in the form of PAMPs. In contrast, subunit vaccines often lack these elements, thus requiring that they be added back.
There is thus a need for new adjuvants, particularly for human vaccines, and for methods for identifying them. It is an object of the invention to provide further and improved adjuvants for use in vaccines and also a directed method for identifying such adjuvants.